This invention relates to fuel cell power production systems and, in particular, to dual-stack molten carbonate fuel cell systems.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
In internally reforming fuel cells, a steam reforming catalyst is placed within the fuel cell stack to allow direct use of hydrocarbon fuels such as methane, coal gas, etc. without the need for expensive and complex external reforming equipment. In a reforming reaction, fuel cell produced water and heat are used by the reforming reaction, and the fuel is internally reformed to produce hydrogen for use in the fuel cell. Thus, the endothermic reforming reaction can be used advantageously to help cool the fuel cell stack.
Two different types of internally reforming fuel cell assemblies have been developed and commonly used. The first type of internally reforming fuel cell assembly is a direct internally reforming fuel cell assembly, in which direct internal reforming is accomplished by placing the reforming catalyst within the active anode compartment. The advantage of direct internal reforming is that the hydrogen produced through such reforming is provided directly to the anode. A second type of internally reforming fuel cell assembly utilizes indirect internal reforming, which is accomplished by placing the reforming catalyst in an isolated chamber within the stack and routing the reformed gas from this chamber into the anode compartment of the fuel cell. The advantage of indirect internal reforming is that the reforming catalyst is protected from poisoning by the fuel cell's electrolyte.
The present state of the art utilizes a hybrid fuel cell assembly with both direct and indirect internal reforming. For example, U.S. Pat. No. 6,200,696 discloses a hybrid fuel cell system that employs both indirect and direct internal reforming with the delivering of the reformed gas from the indirect reforming chamber to the anode flow field.
As can be appreciated, variable loads, powered by a fuel cell have placed varying power demands on the fuel cell during its operation. Accordingly, fuel cells must efficiently handle these varying power demands, while producing sufficient power to satisfy the demands. As a result, in order to increase the fuel cell efficiency and to improve the handling of high and low power demands, fuel cell systems have been proposed in which the excess hydrogen fuel in the anode exhaust (e.g., in a molten carbonate fuel cell, approximately 10 to 50% of the fuel exits the cell as anode exhaust gas) is either combusted for use in heating or cooling applications, or hydrogen is separated for used by a fuel cell or by other devices, or the exhaust is passed to another device that uses dilute hydrogen, such as another fuel cell or an internal combustion engine. In addition, to improve efficiency, some fuel cell systems extract a portion or all of the hydrogen from the anode exhaust, and recycle the extracted hydrogen fuel back to the anode input of the fuel cell.
U.S. Pat. No. 5,413,878 to Williams, et al. discloses a fuel cell system that includes a plurality of fuel cell stacks connected in series so that the separate electrode flows are networked in a serial co-current, serial countercurrent or a combination of serial and parallel flows. In the Williams, et al. patent, each of the fuel cells stacks is an internally reforming molten carbonate fuel cell stack, and the anodes of the fuel cell stacks are connected in series so that anode exhaust from a first fuel cell stack is passed to an anode of a second fuel cell stack, and so that anode exhaust from the second fuel cell stack is passed to an anode of the third fuel cell stack, and so on.
U.S. Pat. No. 4,917,971, assigned to the same assignee herein, and application Ser. No. 10/860,740, also assigned to the same assignee herein, disclose another power production system, which includes a high temperature fuel cell, such as a molten carbonate fuel cell, and a low temperature fuel cell connected in series, such that anode exhaust from the high temperature fuel cell is conveyed to the low temperature fuel cell for generating additional power and increasing efficiency. In the '971 patent, the anode exhaust from the high temperature fuel cell is cooled and shifted to convert CO and water in the anode exhaust to CO2 and H2, and in the '740 application, water is removed from the anode exhaust so as to increase the concentration of fuel in the exhaust before passing the anode exhaust to the low temperature fuel cell.
The systems disclosed in the Williams, et al. patent, the '971 patent and in the '740 application increase the fuel utilization, and hence, the operating efficiency, of the fuel cell system. However, the additional fuel cell stacks used in these systems for utilizing the unspent fuel in the anode exhaust of the first fuel cell stack result in significant increases in equipment and maintenance costs. The equipment cost of using low temperature fuel cell stacks in the '971 patent and in the '740 application instead of a single stack often outweighs the operating and fuel utilization efficiencies of multi-stack assemblies.
It is therefore an object of the invention to provide an improved dual-stack fuel cell system with improved efficiency.
It is a further object of the present invention to provide a dual-stack fuel cell system with an improved power output at higher efficiency operation.